A hydraulic system applied to an excavator supplies some of a hydraulic fluid discharged from a plurality of variable displacement hydraulic pumps to left and right traveling motors through the shift of traveling spools, and supplies the remaining hydraulic fluid to a boom cylinder through the shift of work apparatus spools to drive a work apparatus such as a boom.
In the case where a low traveling speed is selected by an operator, a low-speed traveling selection signal is applied to a controller to cause the traveling mode of a solenoid valve to be converted into a low speed traveling mode in response to a control signal from the controller. Thus, a signal pressure passing through the solenoid valve is supplied to a speed conversion valve mounted in the traveling motor to drive the speed conversion valve so that the swivel angle of a swash plate of the traveling motor can be maximized to perform a low-speed traveling mode.
In a state in which the low-speed traveling mode is selected, when the two-way traveling operation is performed, the hydraulic fluid discharged from the hydraulic pump is respectively supplied to the left-traveling motor and the right-traveling motor via the traveling spools.
In this case, when the work apparatus is manipulated by the operator (i.e., a combined operation of the two-way traveling operation and the operation of the work apparatus is operation), some of the hydraulic fluid discharged from the hydraulic pumps is supplied to the traveling motors and the remaining hydraulic fluid is supplied to the boom cylinder to drive the work apparatus, and thus the traveling speed is suddenly reduced. For this reason, the operator feels a shock, thereby lowering manipulability and thus degrading workability.